Because the rules don't cover this—not even in the General Guidelines and Glossary on Multiplying (Player's Handbook 304)—, when order isn't specified, this DM has always allowed benefits to be applied in the order that's most favorable to the affected game element. This DM probably uses this guideline—rather than some other method—because the D&D Main FAQ says
As a general guideline, whenever the rules don't stipulate an order of operations for special effects (such as spells or special abilities), you should apply them in the order that’s most beneficial to the "controller" of the effect. (51)
This information is repeated almost verbatim in a different exchange later in the FAQ (113). However, note that when applied generally this is taken totally out of context. (The first FAQ exchange asks if poison damage occurs before or after an assassin uses the special ability death attack, and the second asks if a creature applies energy resistance before or after vulnerability.) Also, many look at the FAQ with a jaundiced eye.
Anyway, with this in mind, a sailing ship that possesses a streamlined hull (Salt and Sea Dogs 71), hoists stunsails (72), and that incorporates a lightning turbine (Arms and Equipment Guide 50) (90,000 gp; 300 lbs.) would see its sailing speed in this DM's campaigns increased first by +5 ft. due to the stunsails then doubled due to the streamlined hull then—during a storm—doubled again due to the effect of the lightning turbine with a minumum speed of 90 ft. due to the turbine.
(This DM also considers speed a real-world value: "When applying multipliers to real-world values (such as weight or distance), normal rules of math apply instead" (PH 304) of doubling twice becoming tripling like with a weapon dealing damage.)